Itoshiki Mirai ni Kuroi Tsurugi wo

Upon turning 18, Third Prince Lucas was betrayed by his fiancée and witnessed his personal knight, Licht, killed before his eyes. Overwhelmed by grief at the loss of someone dear, Lucas cried out in anguish, plunging into the depths of despair... When he opens his eyes again, he finds himself back in the “world before the incident”!? Faced with the same gentle Licht as always, Lucas is overcome with love and tells him he's dismissing him from the position of his personal knight to prevent history from repeating itself. But suddenly, Licht's eyes change, and he seizes Lucas's lips—“If I'd known this would happen, I should've claimed you as mine sooner.”

(Source: Tokyomangasha, translated)